Mass Movement: Definition, Types & Examples

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  • 0:00 What Is Mass Movement?
  • 1:45 Types And Examples Of…
  • 3:20 Lesson Summary
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Lesson Transcript
Instructor: Kimberly Schulte

Kimberly has taught at the university level for over 17 years.

Mass movement involves the movement of material such as soil, rock, mud or snow, down a slope under the influence of gravity. In this lesson, we will look at what causes mass movement, the types of mass movement and see some real life examples of each.

What is Mass Movement?

Mass movement, often called mass wasting, is the downslope movement of a mass of surface materials, such as soil, rock or mud. This mass movement typically occurs along hillsides and mountains due to the influence of gravity and can happen very slowly or very quickly.

Mass movement can occur due to a variety of reasons. The most basic reason is the angle of repose or slope of the hillside. If the angle is overly steep, gravity will pull the material downward, causing a mass movement. The angle of repose can also influence how fast the material will move. When you roll a ball down a gentle slope it moves relatively slow; however, if you increase the slope or the angle, the ball will roll down the incline much faster. Earthquakes are also a common cause of mass movement. As the ground shakes, due to the energy released during the earthquake, portions of the hillside or mountain can come loose and move downslope. The lack of vegetation can also contribute to mass movement. Vegetation helps anchor the soil in place, which prevents it from moving. When vegetation is removed, that anchor is lost and soil can be easily dislodged. An overabundance of water will also make the soil very mobile. Water actually lubricates the soil and contributes additional weight, just like how your clothes are heavier when they get wet. The additional weight helps the material move downslope. Geology also plays an important role. This includes the rock type present, the dip of the rock layers, or the structural nature of the area. While all of the above can cause mass movement to occur, the ultimate control of mass movement is gravity. Without the help of gravity, there would be no downward movement regardless of the cause.

Types and Examples of Mass Movement

Flows occur when the material, soil, and/or rock, behave more like a liquid or fluid. Flows include mudflows, debris flows or lahars (superheated water that moves down an erupting volcano). Flows occur due to a large amount of water or ice present in the soil or material. Flows are most often the fastest traveling and can have speeds in excess of 70 miles per hour, depending on the location and steepness of the slope.

A debris flow in the Philippines.

Slumps behave differently than flows. Slumps occur as a wedge or slice of material that moves as one piece along a curved surface. As a result, it often can resemble a spoon scoop. Slumps typically occur where there is loose material or rock.

Here you can see the typical spoon scoop shape of a slump in Lincoln County, Wyoming.

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